Evaluating out-of-field doses during radiotherapy of paediatric brain tumours using lead shielding and flattening-filter free beams.

Paediatric radiotherapy comes at the expense of increased risk of late effects due to out-of-field dose caused not only by the treatment itself but also by image guidance. This study examined how the out-of-field dose to selected radiosensitive organs was affected by applying a 1 mm lead shielding during delivery of volumetric-modulated arc therapy (VMAT) for paediatric brain cancer. The study also investigated how the out-of-field dose to the same organs was affected by the use of flattening-filter free (FFF) beams. Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms equivalent to a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams and delivered using a Varian TrueBeam linear accelerator, with and without lead shielding applied to the phantoms. The measured out-of-field doses were as large as 200,9 cGy for the whole treatment, with associated secondary cancer risk being as large as 1,1%. Shielding of the phantoms was found to decrease the out-of-field dose by up to 24%. The use of 6 MV FFF beams yielded a decrease in the dose to the testes by 21-42% compared to 6 MV, while in one case increasing the dose to the thyroid by 18%. The observation that only doses to organs distant to the primary irradiated volume were significantly decreased for FFF can be explained by an increase in internal scatter caused by the softer energy spectrum of the Varian FFF beam.

Evaluation of methods for selecting the midventilation bin in 4DCT scans of lung cancer patients.

BACKGROUND: In lung cancer radiotherapy, planning on the midventilation (MidV) bin of a four-dimensional (4D) CT scan can reduce the systematic errors introduced by respiratory tumour motion compared to conventional CT. In this study four different methods for MidV bin selection are evaluated. MATERIAL AND METHODS: The study is based on 4DCT scans of 19 patients with a total of 23 peripheral lung tumours having peak-to-peak displacement ≥ 5 mm in at least one of the left-right (LR), anterior-posterior (AP) or cranio-caudal (CC) directions. For each tumour, the MidV bin was selected based on: 1) visual evaluation of tumour displacement; 2) rigid registration of tumour position; 3) diaphragm displacement in the CC direction; and 4) carina displacement in the CC direction. Determination of the MidV bin based on the displacement of the manually delineated gross tumour volume (GTV) was used as a reference method. The accuracy of each method was evaluated by the distance between GTV position in the selected MidV bin and the time-weighted mean position of GTV throughout the bins (i.e. the geometric MidV error). RESULTS: Median (range) geometric MidV error was 1.4 (0.4-5.4) mm, 1.4 (0.4-5.4) mm, 1.9 (0.5-6.9) mm, 2.0 (0.5-12.3) mm and 1.1 (0.4-5.4) mm for the visual, rigid registration, diaphragm, carina, and reference method. Median (range) absolute difference between geometric MidV error for the evaluated methods and the reference method was 0.0 (0.0-1.2) mm, 0.0 (0.0-1.7) mm, 0.7 (0.0-3.9) mm and 1.0 (0.0-6.9) mm for the visual, rigid registration, diaphragm and carina method. CONCLUSION: The visual and semi-automatic rigid registration methods were equivalent in accuracy for selecting the MidV bin of a 4DCT scan. The methods based on diaphragm and carina displacement cannot be recommended without modifications.

Artifacts in conventional computed tomography (CT) and free breathing four-dimensional CT induce uncertainty in gross tumor volume determination.

PURPOSE: Artifacts impacting the imaged tumor volume can be seen in conventional three-dimensional CT (3DCT) scans for planning of lung cancer radiotherapy but can be reduced with the use of respiration-correlated imaging, i.e., 4DCT or breathhold CT (BHCT) scans. The aim of this study was to compare delineated gross tumor volume (GTV) sizes in 3DCT, 4DCT, and BHCT scans of patients with lung tumors. METHODS AND MATERIALS: A total of 36 patients with 46 tumors referred for stereotactic radiotherapy of lung tumors were included. All patients underwent positron emission tomography (PET)/CT, 4DCT, and BHCT scans. GTVs in all CT scans of individual patients were delineated during one session by a single physician to minimize systematic delineation uncertainty. The GTV size from the BHCT was considered the closest to true tumor volume and was chosen as the reference. The reference GTV size was compared to GTV sizes in 3DCT, at midventilation (MidV), at end-inspiration (Insp), and at end-expiration (Exp) bins from the 4DCT scan. RESULTS: The median BHCT GTV size was 4.9 cm(3) (0.1-53.3 cm(3)). Median deviation between 3DCT and BHCT GTV size was 0.3 cm(3) (-3.3 to 30.0 cm(3)), between MidV and BHCT size was 0.2 cm(3) (-5.7 to 19.7 cm(3)), between Insp and BHCT size was 0.3 cm(3) (-4.7 to 24.8 cm(3)), and between Exp and BHCT size was 0.3 cm(3) (-4.8 to 25.5 cm(3)). The 3DCT, MidV, Insp, and Exp median GTV sizes were all significantly larger than the BHCT median GTV size. CONCLUSIONS: In the present study, the choice of CT method significantly influenced the delineated GTV size, on average, leading to an increase in GTV size compared to the reference BHCT. The uncertainty caused by artifacts is estimated to be in the same magnitude as delineation uncertainty and should be considered in the design of margins for radiotherapy.

Deviations in delineated GTV caused by artefacts in 4DCT.

BACKGROUND AND PURPOSE: Four-dimensional computed tomography (4DCT) is used for breathing-adapted radiotherapy planning. Irregular breathing, large tumour motion or interpolation of images can cause artefacts in the 4DCT. This study evaluates the impact of artefacts on gross tumour volume (GTV) size. MATERIAL AND METHODS: In 19 4DCT scans of patients with peripheral lung tumours, GTV was delineated in all bins. Variations in GTV size between bins in each 4DCT scan were analysed and correlated to tumour motion and variations in breathing signal amplitude and breathing signal period. End-expiration GTV size (GTVexp) was considered as reference for GTV size. Intra-session delineation error was estimated by re-delineation of GTV in eight of the 4DCT scans. RESULTS: In 16 of the 4DCT scans the maximum deviations from GTVexp were larger than could be explained by delineation error. The deviations were largest in the bins adjacent to the end-inspiration bin. The coefficient of variation of GTV size was significantly correlated to tumour motion in the cranio-caudal direction, but no significant correlation was found to breathing signal variations. CONCLUSION: We found considerable variations in GTV size throughout the 4DCT scans. Awareness of the error introduced by artefacts is important especially if radiotherapy planning is based on a single 4DCT bin.